1. Field of the Invention
The present invention relates to an electrostatic copying apparatus such as an electrophotographic copying apparatus.
2. Description of the Prior Art
Generally in an electrophotographic copying apparatus, an image is formed by the use of a photoconductive drum. In the state where the photoconductive drum is uniformly charged by a charging device so as to have a specified potential on a surface thereof, the photoconductive drum is exposed by a light emitted from a light source and reflected on a background of a document. An exposed portion of the surface, which corresponds to the background of the document, is turned conductive so as to allow the potential thereof to be lowered. As a result, an electrostatic latent image corresponding to an image of the document is formed on the photoconductive drum. A portion of the surface which is not exposed by the light and therefore has a high potential is developed with a toner which is charged to have a polarity opposite to that of the photoconductive drum. A toner image thus formed is transferred onto a recording paper and is then fixed, whereby a copy image is formed.
In the case that the background of the document has a poor reflectance, as in the case of a newspaper, there is a possibility that the potential of the exposed portion of the surface of the photoconductive drum corresponding to the background of the document is not sufficiently lowered by the light reflected on the background, thereby undesirably allowing the toner to be adhered on the exposed portion. Such a phenomenon results in an unclear image having a so-called fog. This phenomenon is avoided by heightening the intensity of the light emitted from the light source so as to lower more certainly the potential of the exposed portion corresponding to the background.
Recently, the optical system in the copying apparatus has been provided with a red filter in order to enhance the photosensitivity of the photoconductive drum to a red image of the document.
In a copying apparatus having the red filter in the optical system, the light emitted toward the photoconductive drum is partially shielded by the red filter, thereby lowering the intensity thereof. Accordingly, the potential of the exposed portion corresponding to the background of the document is not sufficiently lowered, which allows the toner to be adhered on the exposed portion. As a result, an image which is unclear due to a fog is formed on the recording paper. Especially in the case that the photoconductive drum is rotated at a high speed for the purpose of a high speed image formation, the intensity of the light emitted toward the photoconductive drum is further lowered, thereby increasing the possibility of a fog being generated.
Even in a copying apparatus constructed to heighten the intensity of the light emitted from the ground with a poor reflectance, the red filter lowers the light intensity, thereby not allowing the potential of the surface to be sufficiently lowered.